Heat oven to 350 degrees F.
In the bowl of an electric mixer, mix together the butter and sugar until creamy.
Reduce speed to low and add the flour, cornstarch and salt. Mix until well blended.
Line a 9 x 9 inch pan with aluminum foil. Spray the foil with cooking spray. Press dough onto bottom of baking pan. Bake for 23-25 minutes or until lightly golden.
Set aside to cool.
Pour caramel over the top of the shortbread. Set it aside to firm up just a little.
Meanwhile, pour the chocolate chips into a microwave-safe bowl. Microwave on high heat for 1 minute, stirring every 20 seconds, or until melted and smooth.
Let the melted chocolate sit for 5 minutes, then spread over the top caramel layer.
Place the pan in the freezer for 20 minutes.
Remove the candy from the pan by lifting the foil lining straight up. Place the candy on a cutting board, and using a hot knive, slice into 12 equal pieces.
Store in refrigerator for up to 3 days. Let stand at room temperature for 10 minutes prior to serving.